Our lab works almost exclusively on CNS regionalization, with a particular focus on the nature and significance of developmental compartments. The formation of cell-tight compartments and their maintenance by lineage restriction boundaries has long been recognised as a key feature of insect development, where the more important functions of the boundaries include maintaining the position of local signalling centres and the prevention of cell mingling, which might disrupt emergent spatial organization in an otherwise homogeneous tissue.
Our earlier work revealed the existence of compartments also in the vertebrates, specifically in the hindbrain region of the CNS (Lumsden and Keynes, 1989; Fraser et al., 1990), where the cell segregation mechanisms involved in their formation and maintenance are still being elucidated - the Eph receptors and ephrin ligands that are expressed selectively in odd- and even-numbered rhombomeres are thought to be responsible for effecting cell lineage restriction, but we know very few candidate regulatory genes that would be required upstream of these realizators.
To illuminate this process, we are using Affymetrix gene chip technology to profile the transcriptomes of individual rhombomeres at the stage of initial segmentation. Interesting transcripts identified thus far include those of a number of transcription factors that have alternate expression, in either the odd rhombomeres or the evens. Functional studies on some of these genes are in progress.
Early rhombomeric pattern and the ontogeny of basal rhythmogenic circuitry
In collaboration with J. Champagnat and G. Fortin (CNRS, Gif-sur-Yvette), we have found a striking relationship between early rhombomeric pattern and the ontogeny of basal rhythmogenic circuitry, whereby development of primordial rhythm generators in each of the even rhombomeres (2, 4 and 6) depends on interactions with their posteriorly adjacent odd rhombomeres (Coutinho et al., 2004). This broadens the significance of hindbrain segmentation beyond modular anatomical organization to the level of network assembly and function.
It has been suggested that the forebrain also is a segmented region of the neuraxis, with a compartmental organisation of prosomeres analogous to the rhombomeres. However, our data on molecular marker expression and the dispersal of labelled cell clones in chick are not in agreement with the 'prosomere model' (Larsen et al., 2001). Nevertheless, we have found that one of the putative interprosomeric boundaries, the zona limitans intrathalamica (ZLI) that bisects the diencephalon transversely, is not a simple boundary but an individual compartment in its own right - the only compartment so far detected in the avian embryo forebrain (Zeltser et al., 2001). The ZLI expresses the secreted factor Sonic Hedgehog (Shh) suggesting that it may also have an important local signalling function; however, genetic disruption of Shh in mouse has failed to reveal a ZLI-specific role for this gene due to severe early developmental defects.
To modulate Shh signalling in a spatiotemporally restricted manner we have performed gain and loss of function experiments in chick embryos using in ovo electroporation and found that Shh signalling is required for region-specific gene expression in thalamus and prethalamus, the major diencephalic brain areas flanking the ZLI. We have further shown that differential competence between thalamic and prethalamic anlagen to respond to Shh signalling is regulated by the Iroquois family transcription factor Irx3, which is expressed only in the presumptive thalamus (Kiecker and Lumsden, 2004). As in Drosophila, compartition is used in the vertebrate forebrain to stabilize the position of a local signalling centre - in this case, a novel structure that regulates the identity of major diencephalic regions.